Maleic anhydride is used as a raw material for products ranging from agricultural chemicals, paints, paper sizing and food additives to synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed, the most successful of which involves the vapor phase oxidation of n-butane to maleic anhydride in the presence of a vanadium-phosphorus-oxygen (VPO) catalyst. Since the development of this method in the 1970's, research has continued to continuously improve the reaction conditions and, particularly, the VPO catalysts.
A review of the improvements made in this technology is given by G. J. Hutchings, in Applied Catalysis, Elsevier Science Publishers B. V. Amsterdam, 72(1991), pages 1-31. The preferred method of preparation of VPO catalysts is the hydrochloric acid digestion of V.sub.2 O.sub.5 and H.sub.3 PO.sub.4 in either an aqueous solvent or non aqueous solvent, such as methanol, tetrahydrofuran (THF) or isobutanol, followed by solvent removal to give what is termed the catalyst precursor, which is then activated by heating. Vanadium, phosphorus and oxygen can form a large number of distinct compounds which have been well characterized, e.g., .alpha.-VOPO.sub.4, .gamma.-VOPO.sub.4, VOHPO.sub.4, (VO).sub.2 P.sub.2 O.sub.7, VO(PO.sub.3).sub.2 and VO(H.sub.2 PO.sub.4).sub.2, the most active catalytic phase believed to be (VO).sub.2 P.sub.2 O.sub.7. While the predominant oxide phase in VPO catalysis is (VO).sub.2 P.sub.2 O.sub.7, the VPO catalysts are usually referred to as "mixed oxides" in recognition of the probable presence of other oxide phases. VPO catalysts have V:P atomic ratios typically in the range of 1:1-1.2 and average, bulk vanadium oxidation states in the range 4.0-4.3. One of the major methods employed to improve the performance of VPO catalysts involved the use of promoters.
In general the methods of preparing promoted catalysts are the same as those described for the unpromoted catalysts. Promoter compounds can be added either (a) together with the vanadium and phosphorus compounds prior to the preparation of the catalyst precursor, or (b) by impregnation of the catalyst precursor prior to formation of the final catalyst by heat treatment. Vanadium/phosphorus/silicon catalyst compositions made in an organic medium are known. Also known are processes for preparing high surface area VPO catalysts. Such catalysts containing up to 0.2 mole, per mole of vanadium, of a transition, alkali or alkaline earth metal, for example, tantalum, titanium, niobium, antimony, bismuth or chromium have also been disclosed. An improved VPO catalyst containing the promoter comprising silicon and at least one of indium, antimony, and tantalum has also been taught.
A number of cations, e.g., Co, Fe, Li, Zn, Ce, Mn, Sn and Lu, are believed to form solid solutions in (VO).sub.2 P.sub.2 O.sub.7. U.S. Pat. No. 4,337,173 discloses a promoted VPO catalyst comprising a substitutional solid-solution type crystalline oxide represented by the general formula [(V.sub.1-x-y-z Fe.sub.x Cr.sub.y Al.sub.z)O].sub.2 P.sub.2 O.sub.7, wherein 0.ltoreq.x.ltoreq.0.40, 0.ltoreq.y.ltoreq.0.40, 0.ltoreq.z.ltoreq.0.40, and x+y+z.ltoreq.0.40, which has the same crystal structure as (VO).sub.2 P.sub.2 O.sub.7. Promoters which these authors propose could be in solid solution with the (VO).sub.2 P.sub.2 O.sub.7 phase appear to be effective in very low concentrations.
In spite of the progress in catalyst and process development over the years, a need still remains to continue to improve the VPO catalyst and it is to that end that this invention is directed.